1. Field of the Invention
The present invention generally relates to optical communications and communications with one or more remote nodes. More particularly, the present invention relates to a method and system for controlling a remote node in a Time Division Duplex (TDD) optical repeater.
2. Description of the Related Art
The drastic development of computer, electronics, and communications technologies is a driving force behind the growth of a variety of wireless communication services being provided over a wireless network. The basic wireless communication service is wireless voice call service to mobile users. The wireless voice call service is typically provided irrespective of a time and location, and text service, which is supplementary to the wireless voice call service, has become increasing popular. Recently, wireless Internet service has emerged to provide wireless Internet access to mobile users.
Along with the development of the information and communications technology, 3rd Generation (3G) International Mobile Telecommunication-2000 (IMT-2000) standardized by the International Telecommunication Union (ITU) Radio communication sector (ITU-R), such as Code Division Multiple Access (CDMA), Evolution-Data Only (EV-DO), and Wideband CDMA (WCDMA), has been deployed. IMT-2000 is a mobile communication system that aims to provide a variety of application services by enabling direct roaming all over the world. The system permits an improved degree of service, thereby permitting personal mobility and service mobility, while ensuring communication quality near or at the level of wired phones, providing high-speed packet data service, and converging wired and wireless networks. Besides increasing voice quality and Wireless Application Protocol (WAP) service quality, IMT-2000 can provide various multimedia services like Audio On Demand (AOD) and Video On Demand (VOD) at higher rates than known by using conventional (i.e. legacy) mobile communication systems.
Moreover, legacy mobile communication systems have limitations in their effectiveness in providing ultra high-speed wireless Internet service because of high wireless Internet fees incurred by the high cost of installing Base Stations (BSs), as well as the limited content generally available for display by the relatively small-size screens of mobile terminals. Wireless Local Area Network (WLAN) is not feasible for providing public service due to propagation interference and narrow coverage. In this context, Wireless Broadband (WiBro) and 4th Generation (4G) wireless mobile communications have been proposed to provide the ultra high-speed wireless Internet service at a significantly lower price than known before, while ensuring portability and mobility.
Compared to CDMA and WCDMA, WiBro and 4G wireless mobile communications use a mobile Internet technology that adopts Time Division Duplexing (TDD) as a duplexing scheme and Orthogonal Frequency Division Multiplexing (OFDM) as a modulation scheme.
TDD is a bi-directional transmission scheme in which downlink transmission alternates with uplink transmission in time. TDD offers higher transmission efficiency than, for example, Frequency Division Duplex (FDD) that uses two different frequencies for the downlink and the uplink. TDD is also suitable for asymmetric or bursty application services through a dynamic time slot allocation. Orthogonal Frequency Division Multiple Access (OFDMA)/Time Division Multiple Access (TDMA) is a similar multiple access scheme to TDMA. which allocates all the subcarriers of a total frequency band to one user during a given time period, and then subsequently allocates them to another user during a next time period. OFDMA/TDMA advantageously increases the data rate per bandwidth and prevents multipath interference.
Typically, a mobile communication system divides a mobile communication service area into a plurality of cells and installs a BS at the center of each cell by introducing the concept of frequency reuse in order to expand the coverage area of a mobile communication network. The radiuses of the cells depend on the strength of signals and/or the amount of traffic in the cells. In other words, a cell radius is relatively smaller in a downtown area having a relatively large amount of traffic, whereas a cell radius is relatively larger in a suburban area with a relatively small amount of traffic, so that the traffic does not exceed the processing capacities of wireless BSs that provide mobile communication services to the cells.
Despite these efforts to provide better mobile communication services through the appropriate control of cell radiuses according to frequency reuse and the quantity of traffic therein, there still exists some limitations such as shadowing, in which there are areas where wireless signals cannot propagate, such as underground, the inside of buildings, tunnels, etc. in a downtown area. Installing a plurality of new wireless BSs to overcome shadowing in the shadowing area is neither cost-effective due to the costs of installing, maintaining and repairing such facilities in each of the shadowing areas, is generally unfavorable to cell design. As a solution to the problem of shadowing, mobile communication services can be provided in areas such as underground, the inside of buildings, tunnels, etc. using an optical repeater system in the shadowing area. The optical repeater system typically overcomes the shadowing by transmitting signals on a communication channel allocated to a mother BS from an optical repeater in an optical transmission scheme.
In particular, the use of an optical repeater is preferable to the 3G mobile communication system and the WiBro system, which have small cell radiuses because these systems use high frequencies and thus experience a large path loss, a small diffraction effect, and a large building transmission loss, compared to the 2nd Generation (2G) mobile communication system.
In order to relay a radio signal between a BS and a Mobile Station (MS), the optical repeater should distinguish a downlink signal from an uplink signal. In FDD, the optical repeater identifies the downlink signal and the uplink signal by means of a duplexer, whereas in TDD, it distinguishes the downlink signal from the uplink signal by use of a switch and selectively provides a path for each of the signals. For this purpose, the TDD optical repeater needs a control signal for accurately detecting the starting points of the downlink signal and the uplink signal, switching on/off the switch for the signals, and thus changing a signal path. The TDD optical repeater, which is typically located in or near the area where there is a shadowing problem, can receive the control signal from the BS by an optical cable.
Moreover, the TDD optical repeater should be equipped with a function for generating a switching control signal so as to control the switch by analyzing a transmission frame so that switching can occur between a downlink period and an uplink period. Due to signal transmission through the optical cable, the optical repeater may suffer from time delay during the transmission. Unless the switch control signal is compensated for the time delay of the optical cable, the switch control signal becomes inaccurate, making it difficult to accurately distinguish between the downlink signal and the uplink signal.
One solution to the time delay can be found, for example, in a Korean Patent Publication No. 2006-0010963 entitled “Method and System for Generating Switching Timing Signal for Separating Transmitting and Receiving Signal in Optical Repeater of Mobile Telecommunication Network Using TDD and OFDM Modulation”.
FIG. 1 is a block diagram of a conventional TDD optical repeater. Such repeaters can include a “donor” or “donor unit” and a “remote” or “coverage unit” which can bring wireless signals into a shadowing area such as a tunnel or inside a large building and distribute the signal where reception is needed.
Referring to FIG. 1, in TDD, a main donor 200 transmits a downlink signal to a remote 250 during a predetermined time period and the remote 250 transmits an uplink signal to the main donor 200 during a time period without any downlink signal.
FIG. 2 is a timing diagram illustrating the timings of transmitting the downlink signal and the uplink signal in the conventional TDD optical repeater.
Referring to FIG. 2, during an optical downlink signal transmission via an optical fiber, optical uplink signal transmission does not occur. In other words, the transmissions are mutually exclusive of each other. Because the downlink transmission and the uplink transmission are carried out in TDD, the optical downlink and uplink signals are both transmitted in the same manner, as opposed to, for example, a system where the downlink and the uplink transmissions are transmitted in a different manner.
Now referring back to FIG. 1, upon the generation of a downlink signal at a predetermined time in an Access Point (AP) 110, the downlink signal is amplified in a Low Noise Amplifier (LNA) 205 and converted into an optical signal through electrooptic conversion in an Electro-Optic (E/O) converter 210. The optical signal is transmitted to the remote 250 through a Wavelength Division Multiplexer (WDM) 215 via an optical fiber. For distinguishing an optical downlink signal from an optical uplink signal, wavelength division multiplexing is used.
Upon receipt of the optical downlink signal in the remote 250, an Opto-Electric (O/E) converter 260 converts the optical downlink signal into an electrical signal after processing in a WDM 255. A separator 265 separates the signal so that a portion goes to switching timing signal generator 290 and a portion to High Power Amplifier (HPA) 260. A switch 275, which receives the timing signal generator by generator 290 and the amplified signal from HPA 260 switches the electrical signal to an antenna according to a switching timing signal generated from a switching timing signal generator 290.
Upon receipt of an uplink signal received through the antenna at a predetermined time, the switch 275 switches the uplink signal to an LNA 280 and an E/O converter 285 converts the electrical signal received from the LNA 280 into an optical signal. The optical signal is transmitted to the main donor 200 via the WDMs 255 and 215. In the main donor 200, an O/E converter 220 converts the optical uplink signal to an electrical signal and a High Power Amplifier (HPA) 225 amplifies the electrical signal and transmits the amplified signal to the AP 110.
In the conventional TDD optical repeater such as shown in FIG. 1, no channel is allocated for controlling the on/off of the remote 250 or the amplifiers of the remote 250, i.e. an HPA 270 and the LNA 280 and no channel is allocated to carry information about the status of the remote 250 to the main donor 200. Therefore, the control of the remote 250 is unstable.